Replacing a poorly built sub-floor

Q. I have your book and have been reading your columns for years but have never heard a solution to this problem.

My house is a small walk-out-basement ranch built in 1973. I purchased it five years ago and have been amazed at how poorly it is constructed.

The sub-floor on my house consists of two layers. The first layer is 3/8 inch plywood with a few too many voids. The second layer is ½ inch particle board installed on top around all the walls. Many of the pieces I have seen are about 2 by 3 foot. They are attached to the floor using many small nails. No effort was made to have the pieces end over a joist. Most joists are not plumb by up to 3/4 inch and can vary 1/4 inch in height from one to the other.

The result of the assembly method above is the particle boards are not secure so my floor creaks and have a few ridges where particle board meet. All the flooring in the house is shot so now is the time to fix it.

To fix it I will first finish making the joists plumb. I have already plumbed about 1/3 of the joists and added hangers on the end. I was thinking of pulling up all the particle boards and replacing it with ½ inch Advantech. I would be careful to end each sheet on a joist and be sure the next sheets overlaps that joist. I am not sure if I should glue the two layers together, use screws or nails. What do you think?

A. Sorry you ended up with such a poorly constructed house. It sounds as if whoever built this house had a lot of leftover pieces of particle boards from some other project and a complete lack of building experience.

The small particle board pieces not ending on a joist put the stress onto the 3/8th inch plywood, causing a lot of flexing. Your approach is sound: the joists need to be properly supported at each end; they should all be on the same level and be of the appropriate size and spacing for the span to avoid bouncing when walked on. They should also be braced at mid span; if they are not, consider installing bridging.

If they are undersized, you can reinforce them by sistering new joists to them or, if the spacing is 24-inches on center, installing new joists in the middle of the space. You can leave the 3/8th plywood and cover it with full size Advantech panels. Be sure to stagger all the joints of the Advantech with those of the plywood, and that all joints are on joists.

Make sure that the 3/8th plywood is properly secured with screws every 12 inches or less, and do the same with the Advantech. Good luck with the project; you’ve got it well figured out.